I have to back-step from what I wrote above. While you can argue today slavery was not the cause - a sort of euphemistic looking back - all the issues leading to secession and the war have slavery as the backdrop. Many arguments prior to Lincoln's presidency were about slave free states v slave states and as such formed a mood in which the southern states felt threatened and under attack. If Lincoln hadn't won the presidency, slavery would have continued and the abolition of it would have happened some time in the future. This piece examines the feelings of the people at the time and supports slavery as the prime cause.
"Her conclusion is that the Americans who fought the Civil War overwhelmingly thought they were fighting about slavery, and that we should take their word for it."
http://www.americanheritage.com/rss/articles/web/20070503-civil-war-chandra-manning-slavery.shtml
The issue of enslavement was not why the South or North fought the Civil War. If the US Government had implemented some law or policy to outlaw the practice in the 85 years preceding the war, it might be a valid argument. That didn't happen. If, in these 85 years, Congress or the Supreme Court had determined human enslavement to be wrong, unethical, abhorrent, and illegal, it might be a valid argument. That did not happen! Instead, quite the opposite happened, time and time again.
Southern interest in the new 'free states' was, they were largely the ones who would settle such territories. We have to remember, these were businessmen who sought to use the advantages available to them, one of which, was slave labor. We can look at this through the prism of today and think this was horrible, but in 1861, slaves were legal to own, buy, and sell, and the US Government condoned that. Those who pushed for the 'free states' to be 'slave states' had a vested financial interest in doing so, it had nothing to do with their sentiments toward black people. It was about business. Still, the Southern states did not secede from the Union because of western states and whether they would be 'free' or not.
Slavery may have well been "the backdrop" but the fundamental issue, the main primary reason and justification from the Confederate perspective, was that of State's Rights. Did the Federal Government have the "right" to mandate laws concerning basic property rights of individuals in the states? From a purely Constitutional standpoint, this was a legitimate argument at the time. In fact, it still remains a legitimate argument in America, and we grapple with this all of the time. What 'authority' is allowed to the Federal Government by the Constitution? From the South's perspective, this was an issue worth fighting for, and they did.
I have repeatedly said, and this is true, not a single Confederate soldier who fought in the Civil War, ever owned a slave. Most of them didn't even know someone who owned slaves. Only 2% of the population in the South, owned any substantial number of slaves. The young men who died under the Confederate flag, were not fighting to protect the institution of slavery. They were fighting for a way of government, the concept that states and people retain rights not enumerated to the Federal government. Did this include the issue of 'free states' and 'slave states'? Yes, but it was not THE reason the war was fought. It actually became "the reason" out of what Lincoln deemed "military necessity" when he issued the Emancipation Proclamation. An order which actually didn't free a single slave in the North, only slaves in the South, where the Union had no control or authority. Again, had the Emancipation Proclamation freed ALL slaves, and had Lincoln issued this BEFORE the war, one could make the argument the Civil War was fought over the issue of slavery. That didn't happen.
Another 'argument' that is often presented is, the North fought over the issue of secession, but did you know, the State of New York actually threatened to secede from the Union over the Civil War? A little tidbit that gets lost in the debate, but it did happen. Along with Lincoln's exhaustive attempts to repatriate slaves to another country, acceptance of a 'slave state' into the Union, West Virginia, in 1863, and a litany of 'compromise' proposals by the US Government, which would have allowed slavery to exist well into the next century. So, to argue the war was fought over the issue of 'enslavement' is invalid and incorrect. It simply wasn't.